With increase year by year in demand for premium lubricant base oil all over the world and increasing requirements on environmental protection, there appears a great need for high quality base oils having type II and type III high viscosity index. Despite of being the means primarily used for production of premium lubricant base oil at present, hydrogenation technology is now facing tough challenges, and there is an urgent need to develop a hydrogenation technology with more superior performance, and higher activity and stability in order to adapt to the trend of processing worse raw materials.
The essence of hydrogenation technology lies in highly efficient hydrogenation catalysts to be developed. In the field of development of hydrogenation catalysts for lubricant base oil, highlighted issues include insufficient hydrogenation isomerization performance of the catalyst, poor aromatic ring-opening capability, and low yield of base oil while maintaining the viscosity index and pour point of the base oil. These issues are primarily due to inadequate utilization of active centers of noble metals in the catalyst and undesirable distribution of noble metals therein, which in turn leads to low activity of the catalyst. Moreover, in order to improve catalyst activity, noble metal contents are increased, resulting in a high cost for manufacture of the catalyst.
Currently, it is conventional in the industry to prepare an impregnation solution having a fixed concentration of noble metals and impregnate a catalyst carrier by saturated or oversaturated impregnation, followed by drying and calcination to prepare a catalyst. U.S. Pat. No. 4,399,058 introduces a method for preparing a hydrogenation catalyst in which group VIB and group VIII metal salts are mixed with aqueous ammonia followed by further addition of aqueous ammonia to adjust pH to a certain value so as to prepare a noble metal solution at a fixed concentration, and an inorganic oxide carrier is saturated impregnated with the metal solution, dried and calcinated to provide a hydrogenation catalyst.
By impregnating a carrier using incipient saturated impregnation, the resultant noble metal components usually distribute evenly on the carrier. For catalytic reactions that primarily take place on the catalyst surface, there will be tremendous waste and loss of noble metals, in particular, consequently inevitable increase in the cost for manufacture of the catalyst for expensive noble metals (Pt, Pd, Ru, Rh, Re, Ir, and etc.). Little has been reported on preparation method of hydrogenation catalysts having an uneven distribution of noble metals, in particular a distribution in an increasing gradient.
European patent EP 0 204 314 describes a method for preparing a hydrogenation catalyst with an uneven distribution of noble metal components. In the preparation process, a stepwise multiple-impregnation approach is employed to load the noble metal components, i.e., a carrier is impregnated with a solution of active component A, and then washed, dried, and calcinated; subsequently, it is impregnated with an impregnation solution of active component B, and washed, dried, and calcinated again. By using a preparation method including stepwise washing, drying and calcinating, the noble metal content within the catalyst particle is made higher than the metal content on its surface, and the life span of such catalyst with an uneven distribution is extended as compared to a catalyst with an even distribution. However, due to the complexity of this preparation method and loss of noble metals in the preparation process, the cost for preparation of such a catalyst is rather high.
Chinese patent CN 101927176A discloses a hydrogenation catalyst with a distribution of concentrations of active metals and acid auxiliary in an increasing gradient. In the preparation of the catalyst, inorganic compounds selected from Al2O3 or Al2O3 comprising SiO2, TiO2 and/or ZrO2 are chosen as a carrier, but the introduction of molecular sieves as the carrier is not described. As for the choice of the metals in the catalyst, nonmetal catalysts are used as active components, and molybdenum and/or tungsten and/or nickel and/or cobalt compounds are used as active metals. In the related field of technology, active metal loading in such types of catalysts is known in the art to normally have a mass faction of 10 wt % or more, and it is easy to achieve a distribution of active metals in such a high content in the catalyst in an increasing gradient. However, when noble metals are used as active components, the content thereof in the catalyst is rather low due to the high cost of the noble metals, and it is thus technically difficult to achieve a distribution of concentration of the noble metal components in a low content in the catalyst in an increasing gradient, which is not described in any related patents. Also, this patent is applicable in the preparation and use of catalysts in the field of non-noble metal hydrogenation, and such catalysts are mainly applied in the field of residual oil hydrogenation; whereas, hydrogenation catalysts used in the field of lubricant base oil production are noble metal catalysts. Therefore, the catalyst disclosed in this patent is not applicable in the field of lubricant base oil.